Atomic layer deposition (“ALD”), formerly known as atomic layer epitaxy (“ALE”), is a thin film deposition process that has been used to manufacture electroluminescent (“EL”) displays for over 20 years. See, e.g., U.S. Pat. No. 4,058,430 of Suntola et al., incorporated herein by reference. The films yielded by the ALD technique have exceptional characteristics such as being pinhole free and possessing almost perfect step coverage. Recently, ALD has been proposed for use in the semiconductor processing industry for depositing thin films on semiconductor substrates, to achieve desired step coverage and physical properties needed for next-generation integrated circuits. ALD offers several benefits over other thin film deposition methods commonly used in semiconductor processing, such as physical vapor deposition (“PVD”) (e.g., evaporation or sputtering) and chemical vapor deposition (“CVD”), as described in Atomic Layer Epitaxy (T. Suntola and M. Simpson, eds., Blackie and Son Ltd., Glasgow, 1990).
In contrast to CVD, in which the flows of precursors are static (i.e., flow rates are steady during processing), precursor flows in ALD processing are dynamic. There are many precursor delivery system components, such as mass flow controllers and particle filters, that can be used in CVD processing in which flow resistance and switching speed are not especially important. However, the inventors have recognized that such delivery system components have limited utility in ALD processes and equipment, due to the dynamic precursor flows and fast switching needed in ALD.
Successful ALD growth typically requires the sequential introduction of two or more different precursor vapors into a reaction space around a substrate. ALD is usually performed at elevated temperatures and pressures. For example, the reaction space may be heated to between 200° C. and 600° C. and pumped down to a pressure of approximately 1 Torr. In a typical ALD reactor, the reaction space is bounded by a reaction chamber sized to accommodate one or more substrates. One or more precursor material delivery systems (also known as “precursor sources”) are typically provided for feeding precursor materials into the reaction chamber.
After the substrates are loaded into the reaction chamber and heated to a desired processing temperature, a first precursor vapor is directed over the substrates. Some of the precursor vapor chemisorbs on the surface of the substrates to make a one monolayer thick film. For true ALD, the molecules of precursor vapor will not attach to other like molecules and the process is therefore self-limiting. Next the reaction space is purged to remove excess of the first vapor and any volatile reaction products. Purging is typically accomplished by introduction of an inert purge gas into the reaction space. After purging, a second precursor vapor is introduced. Molecules of the second precursor vapor chemisorb or otherwise react with the chemisorbed first precursor molecules to form a thin film product of the first and second precursors. To complete the ALD cycle, the reaction space is again purged to remove any excess of the second vapor as well as any volatile reaction products. The steps of first precursor pulse, purge, second precursor pulse, and purge are typically repeated hundreds or thousands of times until the desired thickness of the film is achieved.
A key to successful ALD growth is to have the first and second precursor vapors pulsed into the reaction chamber sequentially and without overlap. An ideal set of ALD precursor pulses would be a pair of Delta functions, as illustrated in FIG. 1, which is a simplified timing diagram representing two cycles of a simple ALD process. With reference to FIG. 1, alternating pulses of a first precursor 12 and a second precursor 14 are separated by intervals 16, which can be made small compared to the duration “d” of each of the pulses 12 and 14. For simplicity of illustration, the pulses 12 and 14 are shown in FIG. 1 as having equal duration, but unequal pulse durations would also be feasible.
As noted above, FIG. 1 illustrates an ideal set of precursor pulses. However, in practice, imperfections in the precursor delivery system, precursor adsorption on the walls of the delivery system and reaction chamber, and fluid flow dynamics cause the concentration of precursor material in the ALD reaction space to have a leading edge slope and an exponential decay during purge. FIG. 2 is a simplified timing diagram illustrating respective first and second pulses 22 and 24 in an ALD reactor, each with a leading edge slope 26 and exponential decay 28. With reference to FIG. 2, because the actual pulses 22 and 24 are not Delta functions, they will overlap if the second precursor pulse 24 is started before the first precursor pulse 22 is completely decayed, as illustrated by overlap region 29. If substantial amounts of both of the first and second precursor chemicals are present in the reaction space at the same time, then non-ALD growth can occur, which can generate particles, non-uniform film thickness, and other defects. To prevent the problems caused by non-ALD growth, the pulses 22 and 24 are desirably separated by a purge interval that is long enough to prevent overlap 29.
FIG. 3 illustrates a purge interval 32, between respective first and second precursor pulses 34 and 36, that is sufficiently long to prevent overlap. For simplicity, the purge interval 32 is illustrated as having a duration similar to the duration of the pulses 34 and 36. However, in practice, it is common for purge times in an ALD process to be 10 times longer than the pulse times, due to long exponential decays of precursor pulses caused by flow restrictions and cold spots in the flow path. For example, a ALD process including precursor vapor pulses having a duration of 50 milliseconds (ms) may require pulse intervals of 500 ms or longer to prevent overlap and achieve good film thickness uniformity. Long purge intervals increase processing time, which substantially reduces the overall efficiency of the ALD reactor. The present inventors have recognized that reducing the rise and decay times also reduces the overall time required for each ALD process cycle without causing non-ALD growth, thereby improving the throughput of the ALD reactor.
Conventionally, precursors have been stored and vapors delivered from glass tubes placed inside the reactor, as described in U.S. Pat. No. 4,389,973 of Suntola et al., incorporated herein by reference. The flow of each precursor vapor is controlled by so-called “inert gas valving,” which involves controlling the direction of an inert gas flowing through the tube containing the precursor chemical. Conventional inert gas valving has been employed for about 20 years for the fabrication of EL displays, including its use with certain solid precursors like ZnCl2 and MnCl2. However, the present inventors have found that the particle requirements for other applications, particularly semiconductor processing, are far more stringent than those required for EL display manufacturing. Conventional precursor delivery methods and inert gas valving do not provide a barrier to prevent particles present in powdered precursors from being carried into the reaction space with the pulses of precursor vapor. Further, the conventional methods cannot accommodate certain highly reactive precursors useful for semiconductor processing, which cannot be loaded in an open tube due to their reactivity with air and/or moisture.
For most films grown by ALD, unwanted particles in or on the film will reduce the manufacturing yield. It is therefore important that the precursor delivery system does not emit particles. Preventing particles is especially difficult when one or more of the precursors exist in powdered form at room temperature and pressure. CVD systems commonly include a high efficiency particle filter that can block up to 99.99999% of particles smaller than 0.003 microns. However, the present inventors have found that CVD-type high efficiency particle filters are unsuitable for use in ALD processing because they are highly resistive to flow, which leads to long precursor rise and/or decay times. High efficiency particle filters also have a tendency to become blocked by coarse particles emanating from a supply of precursor material, which can cause system failures and yield losses in manufacturing. A new type of ALD-oriented particle filtering is therefore needed.
The inventors have also recognized a need for improved control of unwanted precursor migration between pulses (during purging).
U.S. Patent Application Publication No. 2001/0042523 A1 of Kesala discloses a reactant gas source contained in a vacuum shell. Liquid or solid reactant matter is held in an ampoule having an opening covered by a high efficiency particle filter. The ampoule is enclosed within a gas-tight container that defines a gas space around the ampoule. An outlet of the gas-tight container leads from the gas space through a second high efficiency particle filter and into the reaction chamber. Pulses of reactant gases are switched by a backflow of inert gas in a line between the second high efficiency particle filter and the reaction chamber. Flow resistance of the second high efficiency particle filter and a capacitance effect of the gas-tight container can cause long decay times for the pulses of reactant.
U.S. Pat. No. 6,270,839 of Onoe at el. discloses a precursor source for a CVD system that does not include a mechanism for pulsing, as required in an ALD system.
Thus the inventors have recognized a need for improved methods and devices for storing precursor materials in a thin film deposition process, conditioning such precursor materials in preparation for deposition, and introducing pulses of vaporized precursor material into a reaction space of a thin film deposition system.